Problem 79 - Chemistry Matter and Change

Question

Write the sign of \(\Delta H_{\text { system for each of the following }}\) changes in physical state. a. \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{s}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(1)\) b. \(\mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{l})\) c. \(\mathrm{CH}_{3} \mathrm{OH}(1) \rightarrow \mathrm{CH}_{3} \mathrm{OH}(\mathrm{g})\) d. \(\mathrm{NH}_{3}(\mathrm{l}) \rightarrow \mathrm{NH}_{3}(\mathrm{s})\)

Step-by-Step Solution

Answer

a. Positive; b. Negative; c. Positive; d. Negative.

Step 1: Understanding Enthalpy Change (\(\Delta H\))

\(\Delta H\), or the change in enthalpy, represents the heat exchanged at constant pressure during a process. A positive \(\Delta H\) indicates that the system absorbs heat (endothermic process), whereas a negative \(\Delta H\) indicates that the system releases heat (exothermic process).

Step 2: Analyze Process (a)

For \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}( ext{s}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}( ext{l})\), the process involves the conversion from solid to liquid, which is melting. Melting is an endothermic process as the substance absorbs heat to overcome the lattice energy. Therefore, \(\Delta H_{\text{system}} > 0\).

Step 3: Analyze Process (b)

For \(\mathrm{H}_{2} \mathrm{O}( ext{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}( ext{l})\), the process involves condensation, where gas changes to liquid. Condensation is exothermic, implying the release of heat. Therefore, \(\Delta H_{\text{system}} < 0\).

Step 4: Analyze Process (c)

For \(\mathrm{CH}_{3} \mathrm{OH}( ext{l}) \rightarrow \mathrm{CH}_{3} \mathrm{OH}( ext{g})\), the process of converting liquid to gas is vaporization. Vaporization requires energy to overcome intermolecular forces, making it an endothermic process. Thus, \(\Delta H_{\text{system}} > 0\).

Step 5: Analyze Process (d)

For \(\mathrm{NH}_{3}( ext{l}) \rightarrow \mathrm{NH}_{3}( ext{s})\), the process involves freezing, where liquid changes to solid. Freezing releases heat as the system settles into a more ordered state, making it exothermic. Therefore, \(\Delta H_{\text{system}} < 0\).

Key Concepts

Endothermic Process

When a system absorbs heat from its surroundings, it undergoes an endothermic process. In these processes, the energy input is necessary to overcome molecular attractions or bonds. For example, during melting, energy is required to disrupt the structure of a solid to transition it into a liquid state.
We can recognize endothermic processes by a positive change in enthalpy (94H > 0), indicating that energy is absorbed. Other common examples include vaporization, where liquids change into gases, and sublimation, where solids turn directly into gases.
Understanding these processes is crucial as they explain how substances behave under various temperature and pressure conditions.

Exothermic Process

In contrast to endothermic processes, exothermic processes release energy, typically in the form of heat, to the surroundings. This release occurs as the system becomes more stable. An exothermic reaction results in a negative change in enthalpy (94H < 0).
A classic example is condensation, where vapor turns into liquid, releasing energy as bonds form. Similarly, freezing, wherein a liquid solidifies, typically releases heat to the environment.

• Exothermic reactions often feel warm or hot to the touch due to the energy release.
• They are essential in everyday phenomena like combustion and cellular respiration.

Phase Change

Phase changes are physical processes where substances transition between solid, liquid, and gas states. These changes involve energy exchanges and are characterized by specific enthalpy changes. For example, when solids melt into liquids or liquids vaporize into gases, energy is absorbed, denoting an endothermic effect.
On the flip side, the transition from gas to liquid via condensation, and liquid to solid through freezing, involves releasing energy, marking exothermic processes.

• Phase changes occur without a change in temperature until the transition is complete.
• Each substance has unique phase transition temperatures, like the melting point and boiling point, which depend on atmospheric pressure.

Melting

Melting is the phase change from a solid to a liquid. During this process, energy is absorbed to break the ordered structure of the solid, turning it into a freer liquid form. This requirement of energy makes melting an endothermic process.
For instance, solid ethanol (94H > 0) requires heat to transition into liquid ethanol. The energy absorbed is used to overcome the tight molecular arrangements found in the solid state.

• Melting points vary among different substances and are influenced by molecular structure and intermolecular forces.

Vaporization

Vaporization is the transformation from liquid to gas. In this process, energy input is necessary to overcome the intermolecular attractions within the liquid, allowing molecules to enter the gaseous phase. Thus, vaporization is endothermic (94H > 0).
Methanol, for instance, takes in heat as it moves from a liquid to a gaseous state. This absorbed energy facilitates molecules in breaking away from the liquid to become vapor.

• Vaporization includes both boiling and evaporation, with boiling occurring at a specific boiling point, while evaporation can occur at temperatures below the boiling point.

Condensation

Condensation is the process where gaseous molecules turn into a liquid state. This transformation involves molecules coming together, releasing energy as they coalesce into a denser phase. Hence, condensation is exothermic (94H < 0).
An example is water vapor in the air turning into liquid on a cold surface, such as a glass window on a chilly day. As the vapor condenses, it releases heat, warming the immediate surroundings.

• Condensation plays a critical role in weather systems, forming clouds and precipitation.

Freezing

Freezing is the phase change from liquid to solid, where a liquid turns into a solid upon losing heat. This process releases energy as the molecules settle into a more ordered structure, marking freezing as an exothermic process (94H < 0).
In the freezing of ammonia, the loss of energy results in the formation of a rigid solid structure. During freezing, the temperature of the substance remains constant until all the liquid has solidified.

• Freezing occurs at a specific temperature known as the freezing point, which is characteristic of every substance.
• It is integral for various natural processes, including the formation of ice in aquatic environments.